1. Field of the Invention
The present application relates generally to the production of amine-aldehyde condensation products, illustratively 3,5-diethyl-1,2-dihydro-1-phenyl-2-propylpyridine, and methods for improving production.
2. Description of Related Technology
Reactive fluid compositions, otherwise known as tough acrylic adhesive compositions or second generation acrylics, are well known adhesive systems. Typically, these are two-part systems that offer the advantages of rapid cure, high strength, and good shelf life.
The first part of these adhesive compositions generally is a polymer-in-monomer solution. Typically, this first part includes and vinyl or acrylic monomers such as methyl methacrylate, and chlorosulfonated polyethylene. Chlorosulfonated polyethylene is a reaction product of a linear or branched polyethylene and sulfonyl chloride. One example of chlorosulfonated polyethylene is HYPALON™ (E. I. DuPont De Nemours' & Co.). Illustrative polymer-in monomer solutions are disclosed in U.S. Pat. Nos. 3,890,407, 3,962,372, 4,106,971 and 4,112,013, herein incorporated by reference.
The second main component of the adhesive compositions is usually a mixture or solution of activators, promoters, initiators, and free radical generators, all of which are discussed in the above-mentioned patents, already incorporated by reference. Examples of typical prior art free radical generators include organic peroxides, hydroperoxides, peresters, persalts, and peracids. Illustrative initiators include amines, such as N,N-dimethylaniline, and illustrative promoters include transition metals such as manganese or nickel.
Amine-aldehyde condensation products are often employed as the activator. Typically, these activators are the products of the reaction between an aliphatic aldehyde (usually C1-12) and an aromatic amine. U.S. Pat. No. 3,890,407 lists illustrative amines and aldehydes useful as condensation reactants. One commonly used activator is the condensation product of the reaction between butyraldehyde and aniline. Commercial sources of the butyraldehyde aniline condensation product include DuPont 808 (E. I. DuPont De Nemours & Co.), VANAX® 808 (RT Vanderbilt Company), Vulkacit 576 (Bayer Corp.), Accelerator 40B (Akrochem Corp.), and NOCCELER 8 (Ocuchishiko Chemical). It is understood that the term “activator” is commonly used interchangeably with the terms “accelerator” and “primer.”
It is known that 3,5-diethyl-1,2-dihydro-1-phenyl-2-propylpyridine (DHP) is the main active ingredient resulting from the condensation reaction of butyraldehyde and aniline. However, the condensation reaction is not a simple addition reaction. Rather, a variety of side reactions occur, with a variety of resulting products. Many such side reactions are discussed in U.S. Pat. Nos. 1,780,334, 1,908,093 and 2,704,750, herein incorporated by reference.
The first known preparation of DHP was reported in Craig, JACS, Vol. 70, pp, 1624–1629 (1948), herein incorporated by reference. The Craig reference misidentified the compound as N-phenyl-3,5-diethyl-2-propyl-1,4-dihydropyridine, but DHP has subsequently been shown to be the principle component of the reaction mixture. As many of the components of the crude mixture were then unknown, the crude DHP mixture has become known as “butyraldehyde aniline condensation product.” There has been surprisingly little information on improvements in the preparation of DHP since the original 1948 publication. Instead, various efforts have focused on distilling DHP from the other components of butyraldehyde aniline condensation product. See U.S. Pat. No. 4,450,050. A commercially available purified DHP, PDHP™ Adhesive Activator (Reilly Industries, Inc.) comprises 85% or more DHP and is produced from purifying a crude butyraldehyde aniline condensation mixture.
One disadvantage to many present adhesive systems that use aldehyde-amine condensation products as surface activators is that the adhesive compositions do not readily adhere to the oily surface of the activator. The adhesive tends to slip or “skate” along the surface of the activator and to flow off the bond area prior to assembly. This “skating” of the adhesive off the activated surface is believed to be caused by a variety of substances that prevent proper wetting and interaction of the adhesive with the surface to be bonded. Due to the nature of the condensation reaction used to make amine-aldehyde activators, many other compounds are formed that do not enhance the activator's ability to function as an accelerator and contribute to the polymerizable portion (polymer-in-monomer part) of the adhesive composition sliding off the surface of an activator-coated part. The result is poor wetting of the surface by the adhesive, resulting in low bond strength. Additionally, many of these noncontributing products impart an obnoxious odor and a brown staining color that can require ventilation and precautionary handling measures and can cause aesthetic imperfections in the surfaces to be bonded. The polymerizable portion of the adhesive composition is generally a clear, viscous material, but when in contact with the activated surface takes on the brown dirty color of the activator.
While highly purified DHP is desirable, purification can be costly, and highly purified DHP is significantly more expensive than its crude counterpart products. Yet materials such as VANAXφ 808, Vulkacit 576, Accelerator 40B, and NOCCELER 8 contain about 40% or less DHP. Condensation methods that result in a product having a greater percentage DHP are desired. Also desired are condensation products that can be purified more readily. More consistent activity can be achieved by higher DHP concentrations and by a reduction in the number and concentration of byproduct impurities that can interfere with the polymerization process. It is also desirable to reduce the levels of impurities that are considered to be hazardous (toxic, flammable, corrosive, etc.).